I'm not sure that accounts for it. I just did a quick test.
One run of Heaven at 1.131v: I get 1.187v in AB and GPU-z stays around 1.131v.
One run of heaven at 1.3v: I get 1.187v in AB and GPU-z, again, stays around 1.131v.
No difference in temperatures either.
What make of 7970 is it?